Acutely ill immobilized patients are at a high risk for thromboembolic events including deep venous thrombosis or pulmonary embolism. Unfractionated heparin (UFH) and low molecular weight heparins (LMWH) are thought to be effective in preventing thromboembolic events. This study is designed to provide efficacy and safety data for thromboprophylaxis with the LMWH certoparin in comparison to thromboprophylaxis with UFH in acutely ill non-surgical patients.